1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image, and a power control method therefor.
2. Description of the Related Art
A previously known image forming apparatus using electro photographic technology is provided with a reading unit that reads an image of an original, a transfer unit that transfers the image of the read original onto a sheet such as a recording paper or a transfer material that is a recording medium, a fixing unit that fixes the transferred toner image to the sheet, a carrying unit that carries the sheet from the transfer unit to a delivery tray via the fixing unit, and a control unit that controls the respective units, etc. The fixing unit has a fixing roller, which is also called a heating roller, to heat and melt toner on a sheet, and a pressure roller that contacts with the fixing roller while giving pressure thereto to pinch the sheet, for example. The fixing roller is formed in a hollow shape and a heating element is held by a holding member on the center axis of the fixing roller.
The heat emitted by the heating element in the fixing roller is uniformly radiated on an inner wall of the fixing roller, and thus a temperature distribution of an outer wall of the fixing roller becomes uniform in a circumferential direction. The outer wall of the fixing roller is heated so that the temperature thereof becomes a fixing temperature (for example, 150 to 200 degrees centigrade) suitable for fixing.
The fixing roller and the pressure roller rotate in opposite directions mutually with contacting while giving pressure to each other, and pinch the sheet to which the toner has been adhered. In a pressure contact position (also referred to as a “nip position”, hereinafter) between the fixing roller and the pressure roller, the toner on a sheet melts with the heat from the fixing roller, and is fixed to the sheet by the pressure given from both the rollers.
Various measures have been proposed as this heating element. For example, U.S. Pat. No. 6,181,905 discloses a fixing unit that adopts an induction heating method (IH method), in which a time required to reach the fixing temperature (a warm-up time) is short and an electricity-to-heat conversion performance is high, using high frequency induction from view points of a user's convenience and of energy saving.
Since the fixing unit of this IH method converts power supplied from AC power source into a high frequency pulse and applies it to a coil, it can control power consumption by changing a pulse width. A time between starting the image forming apparatus and enabling the image formation depends on the warm-up time required to heat the fixing roller to the fixing temperature. Other units become available in relatively short time after the start-up. Therefore, at the start-up of the image forming apparatus, if the power supplied by the AC power is concentrated on the fixing unit as much as possible, the warm-up time can be shortened. On the other hand, at the time of operation of the image forming apparatus, such as a time of image formation, the electric power is consumed by a carrying motor of the carrying unit and a high voltage power source for transferring the toner to a photoconductive medium, the recording medium in the transfer unit, or the like. Accordingly, although the electric power supplied to the fixing unit becomes lower than that at the time of a start-up, the electric power to keep the fixing temperature has to be supplied at the minimum.
Here, the limitation of the maximum electric power consumption of the image forming apparatus is determined by a specification of commercial power source to which the apparatus is connected. For example, the specification of the output port (referred to “a plug socket”, hereinafter) of the 100V commercial power source used in a general office in Japan is 1500 W.
However, when an impedance of a cable wired from a switchboard to a plug socket is large, a passage of a current causes a corresponding voltage drop. Therefore, since a passage of a large current enlarges the voltage drop, there is a possibility that an abnormality occurs in an operation of the image forming apparatus.
Japanese laid-open patent publication (Kokai) No. 2006-293212 (JP2006-293212A) discloses a method for changing an operation mode to a low current mode when the voltage drop of the plug socket is detected. Japanese laid-open patent publication (Kokai) No. 2007-102008 (JP2007-102008A) discloses a method for controlling the power consumption so that the voltage drop falls within a predetermined range when the voltage drop of the power source is detected during operation.
However, the above-mentioned conventional image forming apparatuses have the following problems. A condition of the power source varies depending on an installed location and an operation environment of the image forming apparatus. For example, an impedance of the power source may be high as a result of using a thin cable or by another factor, or the image forming apparatus may share the power source with other machines by branching from the same plug socket.
If large electric power is supplied to the fixing unit in a possible early stage after powering-on in order to shorten a warm-up time of the image forming apparatus, the voltage drop at the time of a start-up may suddenly shut down the power source of the image forming apparatus at a location where the power source condition is poor.
If the method of Japanese laid-open patent publication (Kokai) No. 2006-293212 (JP2006-29312A) or Japanese laid-open patent publication (Kokai) No. 2007-102008 (JP2007-102008A) mentioned above is used, it is possible to change the control method so as to continue the operation, when power source voltage descends during the operation of the apparatus. However, when the power source of the apparatus shuts down at the time of a start-up, the control method cannot be changed by the techniques of the above-mentioned publications. Therefore, when trying to start the apparatus without improving the power source condition, since the power source of the apparatus shuts down every time the apparatus starts up, the apparatus cannot be started. Accordingly, a down time during which the apparatus cannot be used becomes long and thus a user feels distrust and a disadvantage.
As a result of this, it is necessary to give priority to the start-up of the apparatus as possible by changing a starting method so as to shorten the down time, even when the power source condition is bad.